add option to decrypt stored passwords

this might require to add a keystore or (better) integrate with the desktop (unity, gnome-shell, KDE, etc.) if available

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: remmina 1.1.1-1ubuntu1
Uname: Linux 4.0.5-040005-generic x86_64
ApportVersion: 2.17.2-0ubuntu1.1
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Jun 22 12:39:10 2015
InstallationDate: Installed on 2015-06-08 (13 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: remmina
UpgradeStatus: Upgraded to vivid on 2015-06-09 (13 days ago)